1. Field of the Invention
The present invention relates in general to slab style gate valves and, in particular, to a replaceable seal for sealing between a floating gate valve seat and a gate valve body.
2. Brief Description of Related Art
A typical gate valve used in connection with oil and gas production has a body with a flow passage extending through it. The flow passage intersects a central cavity. Seat rings are placed in counterbores formed in the flow passage at the intersection of the flow passage with the cavity. A gate will variably move through the central cavity to block the flow passage. The seal rings bridge a gap between the valve body and the gate to prevent fluid from flowing around the gate when the gate blocks the flow passage.
The seats have seals which seal the seat to the counterbore of the flow passage. These seals prevent the entry of fluid from the flow passage or chamber of the body to the downstream flow passage. When the gate is opened, the seals perform no function. When the gate is closed, the seats may seal the chamber either on the upstream or the down stream side to prevent flow of fluid past the gate. Some seats float in the counterbore between the valve body and the gate. These floating seats may provide improved sealing characteristics as the seat is better able to seal to both the valve body and the gate as the gate moves through the cavity. This is due to the ability of the seat to adjust to the position of the gate in response to movement of the gate through the cavity.
Typical seals are formed of a thermoplastic or elastomer material and do not hold up well under high pressure, extreme high temperature, and extreme low temperature environments. For example, in high pressure environments, the thermoplastic materials do not reliably maintain a seal between the valve body and the seat ring. Similarly, in high temperature or low temperature environments, the thermoplastic seals do not reliably maintain a seal and break down at a more rapid rate. For example, in high pressure and high temperature extremes, elastomer and thermoplastic seals will begin to flake at points of contact with the seat and valve body. The flaking quickly breaks down the seal causing the seals to wear relatively quickly. The quick rate of failure means that the fluid will flow past the seals and prevent the seal and seat assembly from performing it's function. Therefore, seal replacement is a necessity during the life of the gate valve and gate valve seat. A typical seal is integral with the seat. Thus, when the seal is damaged or begins to wear past the point at which the seal is effective, the entire seat must be replaced to replace the seal. Replacing the entire seat is costly, and time consuming.
In order to overcome the problems with elastomeric seals, some gate valves employ metal seals between the gate valve body and the gate valve seats. While metal seals overcome the problems of elastomeric seals, metal seals are unable to be used in a floating seat configuration. This is because metal seals are pressed into sealing engagement between the seat and the valve body in an interference fit. If the seat were to move to maintain contact with the gate valve, the metal seal between the seat and the body would be relieved of the pressure of the interference fit and fail to maintain a seal. In addition, metal seals are again generally integral to the body of the seat, thus when the seal wears out, the entire seat and seal assembly must be replaced at great commitment of time and cost.
Fluid born through gate valves may often contain particles and debris, such as sand. Conventional seals are generally unable to effectively deal with such particles. If particles become wedged between the seal and the seat, or the seal and the body, the particles may allow for fluid to flow past the seal. In addition, such debris and particles may have a negative impact on the ability of the gate and the seal to operate as intended and variably allow passage of fluid downstream.
Therefore, there is a need for a replaceable gate valve seat seal for high pressure environments that provides a reliable seal at both high and low temperatures extremes, while operating as a particle exclusion device.